Processes for conversion of starch to dextrose have long been known in the art. Glucoamylase is an enzyme capable of converting starch to dextrose. The use of glucoamylase for producing dextrose and dextrose-containing syrups is well known in the art. Processes using glucoamylase generally fall into three categories. These are the acid-liquefaction-enzyme conversion process, the enzyme-liquefaction-enzyme conversion process, and the enzyme-solubilization-enzyme conversion process (the granular starch hydrolysis process as disclosed and claimed in U.S. Pat. Nos. 3,922,197; 3,922,198; 3,922,199, and 3,922,200).
In the acid-enzyme process, starch is liquefied and hydrolyzed in an aqueous suspension containing 20 to 40 percent starch and an acid, such as hydrochloric acid. The suspension is then heated to a high temperature, i.e., a temperature between about 70.degree. C. and about 160.degree. C. and at a pH between about 1 to 4.5 to liquefy and partially hydrolyze the starch. The liquefied and partially hydrolyzed starch will generally have a dextrose equivalent (D.E.) value up to about 20 and preferably up to about 15. Typical acid-enzyme processes are disclosed in U.S. Pat. Nos. 2,305,168; 2,531,999; 2,893,921; 3,012,944 and 3,042,584.
In the enzyme-enzyme process, starch is liquefied and partially hydrolyzed in an aqueous suspension containing 20 to 40 percent starch and a liquefying enzyme such as bacterial alpha-amylase enzyme at a temperature of from about 85.degree. C. to about 105.degree. C. The dextrose equivalent value of the liquefied and partially hydrolyzed starch is generally less than about 20 and preferably less than about 15. A process for preparing a low dextrose equivalent partial hydrolysate suitable for converting starch to dextrose and dextrose-containing syrups comprises liquefying starch in water with a bacterial alpha-amylase enzyme preparation to a dextrose equivalent value of from about 2 to about 15, heat treating the slurry containing the liquefied starch to a temperature greater than about 95.degree. C., and thereafter converting the liquefied starch with a bacterial alpha-amylase enzyme preparation to a D.E. of up to about 20. This process is disclosed and claimed in U.S. Pat. No. 3,853,706. In the enzyme-liquefaction-enzyme process, dextrose equivalent values of about 97 are regularly obtained.
In the enzyme-solubilization-enzyme process, a slurry of granular starch is digested by the action of bacterial alpha-amylase (preferably a bacterial alpha-amylase enzyme preparation derived from the microorganism Bacillus licheniformis) under conditions such that some granular starch is present during digestion. The digested starch may be thereafter converted to dextrose or dextrose-containing syrups by other enzymes such as glucoamylase.
The low dextrose equivalent liquefied starch hydrolysates prepared by any one of the three processes mentioned above can then be treated with soluble glucoamylase enzyme preparations to convert the low dextrose equivalent starch hydrolysate to dextrose or dextrose containing syrups.
Glucoamylase preparations are produced from certain fungi strains such as those of genus Aspergillus, for example, Aspergillus phoenicis, Aspergillus niger, Aspergillus awamori, and certain strains from the Rhizopus species and certain Endomyces species. Glucoamylase effects the hydrolysis of starch proceeding from the non-reducing end of the starch molecule to split off single glucose units at the alpha-1,4 or at the alpha-1,6 branch points. Commercial glucoamylace enzyme preparations comprise several enzymes in addition to the predominating glucoamylase, for example, traces of proteinases, cellulases, alpha-amylases, and transglucosidases.
Alpha-amylase enzyme is produced from many types of microorganism, for example by certain Aspergillus species and Bacillus subtilis. Alpha-amylase is an endo enzyme capable of randomly splitting the starch molecule into smaller chain units and is used in the enzyme-enzyme process as liquefying enzyme. Alpha-amylase does not selectively split off dextrose units and breaks only the alpha-1,4 chain link. Debranching enzymes or alpha-1,6-glucosidases have recently been used for their ability to break the alpha-1,6 linkages which cannot be hydrolyzed or broken by the action of alpha-amylase.
Considerable interest has been developed in the use of immobilized enzyme technology to continuously produce dextrose from starch. Various procedures have been described for the immobilization of glucoamylase, alpha-amylase, and amylolytic enzyme combinations.
In the art of enzyme immobilization, glucoamylase immobilization has received considerable attention. Many methods of glucoamylase immobilization are available, for example, in U.S. Pat. Nos. 2,717,852; 3,519,538; 3,619, 371; 3,627,638; 3,672,955; 3,715,277; 2,783,101; and 3,950,222.
Processes using immobilized glucoamylase treatment of starch hydrolysates have recently been reported from Iowa State University. Examples of these processes are the following: Weetall and Suzuki Immobilized Enzyme Technology: Research and Applications, Plenun Press, New York, N.Y. (1975) pp. 169-297; Lee et al. Die Starke 27 (1975) No. 11 pp. 384-387; Lee et al. Biotechnology and Bioengineering, vol. XVII (1976) pp. 253-267; Lee et al. Paper 601, 10th ACS Midwest Meeting, Nov. 7, 1974.
In the Iowa State work, glucoamylase was covalently immobilized on Corning porous silica ceramic carrier. Low dextrose equivalent starch hydrolysates produced by the previously described processes were continuously passed over the immobilized glucoamylase. The maximum dextrose concentration in the product (based on dissolved solids) varied from 87 to 93 percent depending on the dextrose equivalent and the amount of reversion products in the feed. In all examples, yields of dextrose using immobilized glucoamylase were lower than that obtained using soluble glucoamylase on the same substrate.
High dextrose equivalent hydrolysates produced using alpha-amylase resulted in lower dextrose yields than obtained with low dextrose equivalent hydrolysates. For example, in the Die Starke report, when higher dextrose equivalent substrates were substituted for the 24 D.E. material initially employed, dextrose concentration as percent of total dissolved solids decreased from 90.1% to 87.0% for 34 D.E. substrate and 86.6% for 42 D.E. substrate.
German patent application OS 25 38 322 discloses a process for the conversion of starch to dextrose through the use of a combination enzyme system consisting of immobilized glucoamylase and alpha-amylase. The latter enzyme can be soluble alpha-amylase or immobilized alpha-amylase. It is important to note that, during the immobilization of the glucoamylase preparation, the alpha-amylase inherently present therewith becomes essentially inactive. Thus, in order to provide maximum utilization of the immobilized glucoamylase in the conversion of starch to dextrose, it is taught that additional soluble and/or immobilized alpha-amylase must be made available during the conversion in addition to the glycoamylase